An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles dispersed in a dielectric solvent. An EPD typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic fluid composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates.
An electrophoretic fluid may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate may be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color.
Alternatively, an electrophoretic fluid may have two types of pigment particles of contrasting colors and carrying opposite charges, and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. In this case, when a voltage difference is imposed between the two electrode plates, the two types of pigment particles would move to the opposite ends (top or bottom) in a display cell. Thus one of the colors of the two types of the pigment particles would be seen at the viewing side of the display cell.
In another alternative, color pigment particles are added to an electrophoretic fluid for forming a highlight or multicolor display device.
For all types of the electrophoretic displays, the fluid contained within the individual display cells of the display is undoubtedly one of the most crucial parts of the device. The composition of the fluid determines, to a large extent, the lifetime, contrast ratio, switching rate and bistability of the device.
In an ideal fluid, the charged pigment particles remain separate and do not agglomerate or stick to each other or to the electrodes, under all operating conditions. In addition, all components in the fluid must be chemically stable and compatible with the other materials present in an electrophoretic display.
Currently, the pigment particles in an electrophoretic fluid often have a density which is much higher than that of the solvent in which the particles are dispersed, thus causing performance issues, such as poor grey level bistability, vertical driving and settling phenomena.